A funny thing happened on the way to a nuclear renaissance: For the first time in 15 years, operating nuclear plants are being forced to close, and energy companies are scuttling plans for new plants and upgrades to existing ones.

In addition to four closures of nuclear plants so far this year, two other US nuclear plants are at a crossroads, and dozens more at risk of early retirement.

It points to the thwarted promise of a nuclear industry that 10 years ago seemed on the verge of revival, until derailed by cheap energy alternatives, listless energy demand, and renewed safety and regulatory concerns, especially after the 2011 Fukushima nuclear accident.

The risk of a major event on US soil is extremely small, and the benefits of nuclear’s stable, carbon-free energy are too good to ignore, nuclear advocates say. And energy markets aren't set in stone: If natural-gas prices were to rise, nuclear would be a more economic option.